September 12, 2008

Major Histocompatibility Complex and Mate Selection in Africans and Europeans

This is a very important study posted in the freely available PLoS Genetics. The Major Histocompatibility Complex (MHC) has been implicated in attractiveness, and has even been made the focus of dna-based dating.

Unlike many other human traits, say nose length, where people tend to mate with others like themselves, the opposite has been suggested for the MHC. The reasoning goes, that MHC is related to disease-resistance, and couples dissimilar in it will produce heterozygous children that will be more capable of fighting disease.

To test this hypothesis, scientists looked at how dissimilar spouses actually are in the MHC, comparing it with their genome-wide dissimilarity: the question is, do they differ in the MHC more or less than they do across the entire genome.

The answer was surprising. African spouses tended to be similar to each other -compared to random African individuals- indicating that there was a tendency for people to marry those genetically close to them. However, no significant pattern emerged for the MHC, i.e., they didn't tend to marry those who were either similar or dissimilar to them.

Europeans spouses, on the other hand, were more dissimilar to each other in the MHC than random individuals, and more dissimilar in the MHC than in the rest of the genome. So, this is strong evidence for assortative mating of MHC-dissimilar Europeans, but not MHC-dissimilar Africans.

Here is the authors' explanation:

On the other hand, Yoruba couples exhibited a significant genome-wide signature of assortative mating, which is likely to result from socio-demographic processes specific to this population. The Yoruba are still organized in paternal lineages, which are exogamous units [32] and C. Adebamowo, personal communication. Although we do not have specific ethnological data collected with the Yoruba samples to explain our observations, a process in which matrimonial exchanges between genealogically related lineages are more frequent than matrimonial exchanges between genealogically unrelated lineages could have left such a genome-wide signature. On the contrary, for the MHC region, no significant pattern of similarity/dissimilarity was observed, at either the molecular level or the serological level. Several hypotheses can be proposed to explain this observation: firstly, it is possible either that the MHC is not involved in mate choice in this population, or that social factors are relatively more important than the MHC and that the sample size here does not allow detection of MHC effect on mate choice.

In other words, the genetic similarity between Yoruban spouses may be the result of population structure, in which individuals mate within social units, rather than with individuals from the entire population.

On the other hand, the unimportance of the MHC for Yoruban mating can be attributed to either its unimportance for Yorubans, or to the fact that "social factors" rather than "good chemistry" play a bigger role in mate choice. A test on other African populations with different social patterns would, perhaps, resolve this puzzle.

PLoS Genetics doi: 0.1371/journal.pgen.1000184

Is Mate Choice in Humans MHC-Dependent?

Raphaëlle Chaix et al.

Abstract

In several species, including rodents and fish, it has been shown that the Major Histocompatibility Complex (MHC) influences mating preferences and, in some cases, that this may be mediated by preferences based on body odour. In humans, the picture has been less clear. Several studies have reported a tendency for humans to prefer MHC-dissimilar mates, a sexual selection that would favour the production of MHC-heterozygous offspring, who would be more resistant to pathogens, but these results are unsupported by other studies. Here, we report analyses of genome-wide genotype data (from the HapMap II dataset) and HLA types in African and European American couples to test whether humans tend to choose MHC-dissimilar mates. In order to distinguish MHC-specific effects from genome-wide effects, the pattern of similarity in the MHC region is compared to the pattern in the rest of the genome. African spouses show no significant pattern of similarity/dissimilarity across the MHC region (relatedness coefficient, R = 0.015, p = 0.23), whereas across the genome, they are more similar than random pairs of individuals (genome-wide R = 0.00185, p<10−3). We discuss several explanations for these observations, including demographic effects. On the other hand, the sampled European American couples are significantly more MHC-dissimilar than random pairs of individuals (R = −0.043, p = 0.015), and this pattern of dissimilarity is extreme when compared to the rest of the genome, both globally (genome-wide R = −0.00016, p = 0.739) and when broken into windows having the same length and recombination rate as the MHC (only nine genomic regions exhibit a higher level of genetic dissimilarity between spouses than does the MHC). This study thus supports the hypothesis that the MHC influences mate choice in some human populations.

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